The present invention is directed to a method operating in a data or image processing system, such as a digital computer, that restores images scanned in the presence of mechanical vibration, thereby improving the quality of the image. Images scanned in the presence of mechanical vibrations are subject to artifacts such as brightness fluctuation and geometric warping.
In one embodiment of a document scanner, a linear sensor array (LSA) typically passes beneath a document placed upon a platen, ideally, at uniform velocity. The output of a single photosensor in the LSA is proportional to the integrated irradiance received by the sensor as it scans through an exposure region. When vibratory motion of the sensor array is considered, the actual size of the exposure region is time-varying. This introduces artifacts such as brightness fluctuation and geometric warping.
The effect of motion on the resolution of analog imaging systems has been studied. The problem typically addressed concerns characterization of the image blur caused by motion during the integration time of a photosensitive imaging medium. In a typical case the blur is usually constant throughout the image and may be caused by linear or vibratory motion. The blur is constant because the devices commonly acquire the full spatial extent of the image at one time, as in a conventional photographic camera or flash exposure xerographic systems. Characterization of this spatial uniform blur leads to a deconvolution-type image restoration process, for example, S.C. Som, "Analysis of the effect of linear smear on photographic images," Journal of the Optical Society of America, 61,859-864 (July 1971), D. Wulich and N. S. Kopeika, "Image resolution limits resulting from mechanical vibrations," Optical Engineering 26(6), 529-533 (June 1987), and O. Hader, I. Dror, and N. S. Kopeika, "Image resolution limits resulting from mechanical vibrations. Part IV: real-time numerical calculation of optical transfer functions and experimental verification," Optical Engineering 33(2), 566-577 (February 1994). The present invention differs from that body of work in that it is directed to images digitized in a raster or scan line fashion where each line may be acquired with a different arbitrary velocity of the light sensing array.
Heretofore, a number of patents and publications have disclosed methods for correcting or compensating for unwanted mechanical motion in imaging devices, the relevant portions of which may be briefly summarized as follows:
U.S. Pat. No. 5,153,644 to Yang et al., issued Oct. 6, 1992, discloses the correction of image distortion resulting from vibrational or speed distortions between an imaging device and a photoreceptor in a xerographic printing apparatus.
U.S. Pat. No. 4,628,368 to Kurata et al., issued Dec. 9, 1986, teaches a method of controlling the sub-scanning of a document whereby any vibration in the reading section is prevented by controlling acceleration/deceleration in accordance with the status of a data buffer so as to eliminate image irregularity.
In "Redundant Rastering for Decreased Vibration Sensitivity in Digital Printers," Xerox Disclosure Journal, Vol. 15, No. 1, (January/February 1990), pp. 31, R. Loce, R. Melino, and W. Lama disclose the digitization of an image at a low frequency and the subsequent printing at a higher frequency so as to avoid a printer's sensitivity to non-uniform motion at the low frequency.
In "Modular Driver For Image Scanners," Xerox Disclosure Journal, Vol. 18, No. 1, (January/February 1993), pp. 7-9, R. Baran and R. Ryon teach the necessity of reducing uncontrolled carriage motion in a document image scanner.
Several authors have considered the effects of nonuniform photoreceptor motion and other sources of noise in slit scanning photocopiers and digital printers. R. Loce and W. Lama in "Exposure Strobing in Photocopiers," Journal of Imaging Science, 32(6), 238-247 (November/December 1988) characterized the periodic exposure level error caused by vibration in a slit scanning photocopier. F. Bestenreiner, U. Geis, J. Helmberger, and K. Stadler, "Visibility and Correction of Periodic Interference Structures in Line-by-Line Recorded Images," Journal of Applied Photographic Engineering, 2, 86-92 (1976), K. Takiguchi, T. Miyagi, A. Okamura, H. Ishoshi, and F. Shibata, "Effect of Photoreceptor Drum Rotational Speed Variation on Laser Beam Printer Halftone Reproduction," Proceedings of the SPSE Third International Congress: Recent Advances in Non-Impact Printing Technologies, 168-172, San Francisco (August 1986), and D. Haas, "Contrast Modulation in Halftone Images Produced by Variation in Scan Line Spacing," Journal of Imaging Technology, 15, 46 (1989) examined the effects of periodic scan line position errors when printing periodic binary patterns (e.g., halftones). P. Schubert, "Periodic Image Artifacts from Continuous-Tone Laser Scanners," Applied Optics 25(21) 3880-3884 (1986) and R. Firth, D. Kessler, E. Muka, M. Noar, and J. Owens, "A Continuous Tone Laser Color Printer," Journal of Imaging Technology 14, 78 (1988), analyzed banding in continuous tone prints due to periodic errors. P. Burns, M. Rabbani, and L. Ray, "Analysis of Image Noise due to Position Errors in Laser Writers," Applied Optics 25, 2158-2168 (1986), and P. Melnychuck and R. Shaw, "Fourier Spectra of Digital Halftone Images Containing Dot Position Errors," Journal of the Optical Society of America 5(8), 1328-1338 (1988) concentrated on the effect of random errors in the continuous tone and binary cases, respectively. S. Bloomberg and P. Engeldrum, "Color Error due to Pixel Placement Errors in a Dot Matrix Printer," Proceedings of the SPSE Third International Congress: Recent Advances in Non-Impact Printing Technologies, 257-260 (August 1986) analyzed the color error on a print that is caused by random pixel placement errors. R. Loce and W. Lama, "Halftone Banding due to Vibrations in a Xerographic Image Bar Printer," Journal of Imaging Technoloqy, 16(1), 6-11 (1990) employed exposure and xerographic models to examine vibration induced halftone banding in image bar printers.
In accordance with the present invention, there is provided a method performed in an image processor for processing data defining an image scanned in the presence of mechanical vibration to produce a restored output image, the steps comprising:
storing image irradiance data in a first memory, where each item of irradiance data stored therein represents a sample of an area of an original document; PA1 storing, in a second memory, velocity data associated with the image irradiance data, wherein the velocity data reflects the relative instantaneous velocity of a scanning element travelling at a nonuniform rate with respect to the original document; PA1 modeling, using the image irradiance data and velocity data, an approximation function to represent image irradiance at any point in time; PA1 identifying control points as a function of a nominal scanning velocity and a fixed sampling time period; and PA1 resampling the approximation function, using the control points, to determine a restored irradiance level for each area of the original document. PA1 acquiring image data and storing the image data in a first memory; PA1 concurrently acquiring positional data so as to produce instantaneous velocity data associated with the image data and storing instantaneous velocity data in a second memory; PA1 using the instantaneous velocity and image data, reconstructing an underlying model of the irradiance distribution of the original image; and PA1 resampling the underlying model under ideal scanning conditions to produce digital output data defining the original image, wherein the digital output data will be substantially free of artifacts caused by mechanical vibration.
In accordance with another aspect of the present invention, there is provided a method performed in an image input device for producing digital output data defining an original image substantially free of artifacts caused by mechanical vibration within the image input device, the steps comprising:
An object of this invention is to produce an output digital image consistent with a scanner operating under "ideal" or uniform motion conditions. The image restoration method described herein makes use of the relative instantaneous velocity between a linear sensor array and a document being scanned to reconstruct an underlying piecewise constant or piecewise linear model of the image irradiance profile. That reconstructed image is then suitable for resampling under ideal scanning conditions to produce a restored output digital image.
One aspect of the invention is based on the observation of problems with conventional image input scanners and the electromechanical measures adopted in such devices to minimize the impact of vibrations and other nonuniform motion. This aspect is based on the discovery of a technique that alleviates these problems by reconstructing an output image based upon data obtained for the irradiance levels of the image and a motion profile of the scanning array with respect to the document. This technique can be implemented, for example, by a data or image processing apparatus suitable for processing information produced by image input terminals including document scanners, digital copiers and facsimile machines.
The technique described herein is advantageous because it is inexpensive compared to other mechanical approaches directed to minimizing vibration in digital document scanners. In addition, it can be used to reconstruct images produced under nonuniform motion conditions so as to eliminate the effects of the nonuniformity on the irradiance levels recorded for regions of the image.